The scrotum is a sac of skin that hangs from the body at the front of the pelvis, between the legs, and is a male reproductive structure located under the penis. It has the shape of a sac and divides into two compartments, each containing one of the two testes, the glands that produce sperm, and one of the epididymides, where the sperm is stored. The scrotum is responsible for protecting the testes and major blood vessels, as well as tubes that release sperm from the testicles into the penis for ejaculation. It helps with the thermoregulation of the testicles by keeping the temperature of the testis several degrees below the average body temperature, which is an essential factor for sperm production. The scrotum thus protrudes from the body wall, and it contracts from cold, exercise, or sexual stimulation and expands and relaxes when warm. When contracted, it conserves heat, while relaxed, it is smooth and elongated, permitting the circulation of air that effects cooling. The scrotum is a unique anatomical feature of humans and certain other species of land-dwelling mammals.